This invention is concerned with a process for the exchange of isotopes of hydrogen, particularly tritium, between water and hydrogen gas to form a water effluent enriched in said isotopes and a water effluent depleted in said isotopes.
Tritium, which is the heavy radioactive isotope of hydrogen having a mass of 3, makes up a small proportion of the total hydrogen isotopes of normal water. Tritium atoms are substituted for hydrogen atoms in water to form tritiowater, HTO, and tritium oxide, T.sub.2 O. Both these substituted forms are referred to collectively as tritiated water.
The supply of tritiated water is constantly renewed as a result of the action of cosmic rays in the upper atmosphere on normal water vapor whereafter under conditions of atmospheric precipitation, both tritiated water and normal water fall as rain. Nuclear explosions also add to the supply of tritium oxide which is present in rain water. Tritiated water is also produced as a result of controlled nuclear reactions in the presence of normal water.
Removal of substantial amounts of tritiated water from normal water in nuclear reactor effluent and in the effluent from facilities which process nuclear fuels is an important problem for which no satisfactory solution currently exists. In the event of an accidental release of tritium within the "glove boxes" in which nuclear fuels are processed for example, current practice is to take the purge air stream containing the tritium gas into a catalytic reactor to oxidize the tritium and tritiated hydrocarbons to normal water and tritiated water, and then absorb the normal and tritiated water on a desiccant such as molecular sieves. The desiccant is regenerated by heating in the presence of dry helium and the resulting water vapor is then adsorbed on another batch of desiccant at liquid nitrogen temperatures. This latter desiccant when essentially saturated with water, is very highly radioactive and is either disposed of or stored.
There is a need to improve the present process of separating substantial amounts of tritiated water from normal water. At the very least, an improved separation would allow a smaller amount of desiccant to be used in absorbing tritiated water from the gases leaving the first two desiccant beds during the regeneration step of the nuclear reaction effluent. Further, an improved separation would allow tritiated water to be isolated from reactor effluent and even from normal water in a form concentrated enough for sale or other use.
The tritiated water isolated can be used in a number of applications such as in kinetic studies in aqueous media and in biological studies. It is expected that ionized tritium will be used as a reactant with ionized deuterium to form helium in controlled thermonuclear fusion reactors now under development for the production of enormous amounts of energy, since the tritium-deuterium reaction takes place at the lowest temperature of any of the various fusion reactions, i.e. deuterium-hydrogen or deuterium-deuterium.
In the past, several processes have been described for exchanging hydrogen isotopes between liquid water and a hydrogen containing gas. In U.S. Pat. No. 2,690,379 to Urey et al., processes for accelerating hydrogen isotope exchange reactions between liquid water or water vapor and hydrogen gas are desribed using certain supported metal catalysts. The Urey et al. patent describes several counter-current and co-current hydrogen exchange systems which are promoted by those catalysts.
In U.S. Pat. No. 2,787,526 to Spevack, a process for concentrating deuterium in water using liquid water and hydrogen sulfide gas in counter-current flow is provided employing reactors which operate at different temperatures.
In U.S. Pat. No. 3,888,974 to Stevens, a process for hydrogen isotope exchange and concentration between liquid water and hydrogen gas is described. The process comprises contacting between 15.degree. C. and 70.degree. C. in a first catalytic zone, feed water and hydrogen gas from a second catalytic zone, and in counter-current flow contacting at a temperature in the range of 150.degree. C. to 200.degree. C. in a second catalytic zone, pressurized to keep water liquified, the liquid water phase from the first zone and the hydrogen gas withdrawn from the first zone with a catalyst comprising a Group VIII metal. In the first zone, a hydrogen isotope such as deuterium is exchanged into and concentrated in the liquid water in the first zone. In the second zone, the hydrogen isotope is exchanged from the water and concentrated in the hydrogen gas withdrawn from the second zone.
The catalyst used in both reactor zones according to Stevens comprises a catalytically active metal of Group VIII and having an organic resin or polymer coating thereon permeable to water vapor and hydrogen gas and impermeable to liquid phase water. Actual hydrogen isotope exchange does not occur directly between the water in liquid form and the gaseous hydrogen even though liquid water and gaseous hydrogen are brought together in the presence of the catalyst; col. 1, lines 18 to 22.
These prior methods suffer from certain disadvantages. The catalysts of the Urey et al. patent are rapidly deactivated in the presence of liquid water and thus cannot be used in a low temperature reactor with water in the liquid state. The process described in the Stevens patent is chiefly designed for the separation of deuterated water from normal ground water. High pressures are required in the second catalytic zone in order to maintain the water in a liquid state. According to the Stevens patent, the preferred operating temperature of the second catalytic zone is 150.degree. C. to 200.degree. C. For such temperature, the system operating pressure is in the region of about 500 to 6000 psig. These operating conditions are not practical for handling tritiated water because of the risk of leaks and consequent environmental problems. Such is not the case with deuterium which is not radioactive and therefore the engineering problems associated with handling deuterium are significantly less than those associated with tritium. With tritium, all process equipment must be essentially free of leakage. Because the rate of a leak is essentially directly proportional to the pressure, there is need for developing a tritium separation process which can operate at the lowest possible pressure and still achieve efficient exchange of tritium between liquid water and hydrogen gas, preferably not much above atmospheric pressure.
The process of this invention employs at least one high temperature zone and at least one low temperature zone for contacting hydrogen gas and water and is characterized by the following features:
(a) the isotope exchange in the high temperature zone can (1) be carried out at about atmospheric pressure while maintaining all reactants in the gas phase such that a wide selection of catalysts can be easily specified which under these conditions have a longer process life; or (2) can be carried out at relatively low pressures while maintaining water in the liquid phase; PA1 (b) the process permits the use of equipment which can be easily designed for operation of the high and low temperature zones to prevent radioactive tritium leakage from the system; PA1 (c) the tritiated feed water may optionally be split to both the high and low temperature zones, or may be introduced into either the low temperature zone or the high temperature zone; and PA1 (d) the catalysts employed in the zones where liquid water exists are relatively slowly deactivated. PA1 (a) (i) introducing hydrogen gas depleted in tritium, and water in the vapor phase, said water containing tritium, into a first reaction zone, contacting in said zone in co-current flow at a temperature in the range of from about 225.degree. C. to about 300.degree. C. said water vapor and hydrogen gas with a supported metal catalyst, withdrawing from said zone an effluent stream comprising a mixture of hydrogen gas enriched in tritium and water vapor depleted in tritium, and condensing said water vapor to liquid water; or PA1 (a) (ii) introducing hydrogen gas depleted in tritium, and liquid phase water, said water containing tritium, into a first reaction zone, contacting in said zone in counter-current flow at a temperature of about 100.degree. C. said liquid phase water and hydrogen gas with a hydrophobic catalyst comprising a coherent film of a hydrophobic polymer, said film being disposed on a low surface area support and containing at least one finely divided catalytically active metal of Group VIII of the Periodic Table on a high surface area carrier and withdrawing from said first reaction zone a stream of hydrogen gas enriched in tritium and liquid water depleted in tritium; and PA1 (b) introducing said hydrogen gas enriched in tritium withdrawn from said first reaction zone and liquid water containing tritium into a second reaction zone, contacting in said second reaction zone in counter-current flow at a temperature in the range from about 0.degree. C. to about 50.degree. C. said liquid water and said hydrogen gas enriched in tritium with a hydrophobic catalyst comprising a coherent film of a hydrophobic polymer, said film being disposed on a low surface area support and containing at least one finely divided catalytically active metal of Group VIII of the Periodic Table on a high surface area carrier, and withdrawing from said second reaction zone a liquid water effluent stream enriched in tritium and a hydrogen gas depleted in tritium.
The present invention provides a novel and efficient method for the exchange of isotopes of hydrogen, particularly tritium, between water and hydrogen gas to form a water effluent enriched in said isotopes and a water effluent depleted in said isotopes.